Many techniques for measuring pulse waveforms (arterial pulsatile waveforms) are used in the field of medicine. A pulse waveform is analyzed in its shape to be used for examining a circulatory system (such as measuring a degree of arteriosclerosis or measuring stress) and to be used for pulse oximeter (arterial oxygen saturation measurement device) and the like.
Among others, a technique called photoplethysmography, which is a technique for measuring pulse waveforms in a non-invasive and percutaneous manner without blood withdrawal or puncture (See, the following Patent Literature 1, for example), is very widely used since it can easily make measurements without loads on a human body.
A pulse oximeter using photoplethysmography determines oxygen saturation based on measured pulse waveforms by use of a property that oxygenated hemoglobin absorbs infrared light (with a waveform of around 940 nm, for example) and reduction hemoglobin absorbs red light (with a waveform of around 660 nm, for example). That is, the light with the two wavelengths are irradiated on a human body to measure degrees of photoabsorption at the respective wavelengths, a relative concentration of each hemoglobin is calculated based on the resultant degrees of photoabsorption, and oxygen saturation is calculated from the resultant relative concentrations.
There are present two types of pulse oximeter using photoplethysmography including a transmissive pulse oximeter for sandwiching a finger as a site to be measured between a light source and a photo detector, and a reflective pulse oximeter in which a light source and a photo detector are arranged on either side of a site to be measured such as a finger.